In communicating over a telephone line, it is desirable to feed back a portion of the audio signal that is input by the subscriber to his own speaker. This allows the subscriber to hear his own voice. However, if the feedback portion of the audio signal is not properly attenuated it disturbs the subscriber causing him to speak more quietly and thus, sufficient signal levels are not sent. The transmitted audio signal and the incoming received signal are transmitted together over a two-way telephone line. Only a portion of the subscriber's voice signal is fed back to the speaker with the incoming received signal.
Wheatstone bridge circuits have typically been employed in integrated circuits to attenuate the portion of the subscriber's voice that is fed back to the speaker and to provide a portion of the subscriber's voice signal to his own speaker. One disadvantage with the Wheatstone bridge configuration is that it normally utilizes a large number of external components for a subscriber circuit.
In view of the above problems, there exists a need for an anti-sidetone circuit that decreases the required number of external components, applies the maximum transmitted signal to the telephone line and allows easy control of the signal fed back to the subscriber's speaker.